The present invention relates to solid state radiation detectors of the type comprising a photosensitive sensor formed by a plurality of solid state photosensitive elements associated with a radiation converter. The converter converts the radiation that it receives so that it can be exploited by the sensor. The field of use of this type of detector is especially that of medical radiology.
Solid state photosensitive sensors do not react to very short wavelengths such as X-rays for example. In this application, the radiation converter is a scintillator screen that is made out of a substance having the property, when excited by X-rays, of emitting in a range of wavelengths that are greater, namely wavelengths in the visible and near range.
The visible light thus generated is transmitted to the photosensitive elements of the sensor which carry out a photoelectrical conversion of received light energy into electrical signals that can be exploited by appropriate electronic circuits.
In other applications, the converter may be a fluorescent screen and may convert visible radiation into other forms of visible radiation or convert near visible radiation into visible radiation.
The converter can thus for example convert ultraviolet radiation into a visible radiation to which the photosensitive elements of the sensor are sensitive. Other converters receive an infrared radiation which they convert into visible radiation.
The photosensitive elements are made out of semiconductor materials usually crystalline or amorphous silicon. A photosensitive element comprises at least one photodiode, one phototransistor or one photoresistor. This photosensitive element is mounted between a column conductor and a row conductor in order to be addressed. Conductors and the photosensitive elements are deposited on an insulating substrate preferably made of glass.
The assembly is covered, in a way that is standard in the semiconductor field, with a passivation layer that is designed especially to protect the sensor from moisture. This layer is generally made of silicon nitride or silicon oxide.
The example taken is that of the field of X-ray imaging with a scintillator screen as converter. Depending on the application planned, various compositions of scintillating substances are used such as, for example, terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) or again thallium-doped cesium iodide (CsI:Tl).
Crystalline silicon can be obtained only in relatively small dimensions of surface area. It is used to make photosensitive sensors of the charge-coupled device (CCD) type. These CCD type photosensitive sensors are especially used in dental imaging and in mammography.
Hydrogenated amorphous silicon can be used to make larger-sized photosensitive sensors (of up to 50 cmxc3x9750 cm for example). It is generally used, by means of thin-film deposition techniques, to constitute matrices of photodiodes or phototransistors. Variable-sized detection matrices can be applied in all the fields of conventional radiology.
Radiation detectors of this kind using the association of a scintillator screen and of a photosensitive sensor made of semiconductor material are well known, for example from the following documents: J. Chabbal et al., xe2x80x9cAmorphous Silicon X-Ray Image Sensorxe2x80x9d, in the journal SPIE 2708, pages 499-510, 1996; L. E. Antonuk et al., xe2x80x9cDevelopment of a High Resolution, Active Matrix, Flat Panel Imager with Enhanced Fill Factorxe2x80x9d, in the journal SPIE 3032, pages 2-13, 1997; and the U.S. Pat. No. 5,276,329.
Reference may also be made to the French patent application FR-2 605 166 which describes a radiation detector structure using a scintillator screen and a matrix of photodiodes made of amorphous silicon, as well as the working of the radiation detector.
The converter can be deposited directly on the photosensitive sensor. However, the common practice is to make the converter and the photosensitive sensor separately and to couple them by means of a layer of transparent bonder. This is especially the case when the converter is of the xe2x80x9cintensifier screenxe2x80x9d type made of Gd2O2S:Tb for example. However this is also a configuration that can be applied to the case of scintillator screens of the type obtained by evaporation, such as thallium-doped cesium iodide scintillators CsI:Tl which sometimes need to be prepared separately in order so that they can be subjected to the thermal and chemical processes that are incompatible with the photosensitive sensor.
The commonly used bonders are chosen for their properties of adhesion as well as for their flexibility and optical properties. They must withstand mechanical stresses for the radiation detector must be able to withstand mechanical stresses in the form of vibrations and shocks. They must also be transparent to the light produced by the converter.
When the converter is made separately, it is often deposited on a support such as for example an aluminium alloy foil which then forms an input window for the radiation to be detected. These aluminum alloys combine the requisite qualities which are low absorption of incident radiation to be detected for a thickness that gives the foil used sufficient rigidity that is compatible with handling. The sizes of the foils are about 50 cmxc3x9750 cm in the field of general radiology. An absorption of about 1% for a support with a thickness of 100 micrometers exposed to a standard spectrum according to the American standard RQA5 is satisfactory.
The support should be capable of withstanding the temperatures used for the deposition and annealing of the detector. For cesium iodide, this temperature is about 300xc2x0 C. It must be moisture-proof and should be of reasonable cost.
In this configuration, with an electrically conductive support for the converter, there is the capacitive coupling between photosensitive elements if the support is left at a floating potential. This prompts a phenomenon of smearing between photosensitive elements. The signals produced by the highly illuminated photosensitive elements are transmitted to the neighboring photosensitive elements which are not illuminated or hardly illuminated. The result thereof is a loss of contrast at a great distance. It therefore becomes necessary to make the potential of the conductive support fixed for example by giving it a ground connection or taking it to another more appropriate voltage.
It has been noted in certain cases that this detector configuration has a short lifetime.
It is desirable that such radiation detectors should have a lifetime compatible with the period of depreciation of the radiological or other instruments on which they are is mounted, this period being close to about ten years. The radiation detector represents a substantial part of the cost of the machine and it would be better not to have to replace it.
The aim of the invention therefore is to increase the lifetime of solid state radiation detectors.
The inventors, having examined solid state radiation detectors that are out of use, have observed the fact that the conductors and/or the photosensitive elements of the sensors are corroded.
Further investigation has led them to the conclusion that the material of the converter gets partially decomposed in the ambient air and/or under moisture, and produce chemical species that are corrosive for the photosensitive sensor. These species migrate, especially under the effect of the electrical field between the conductors and the support of the sensor, towards the photosensitive elements and the conductors, and they do so despite the passivation layer.
To improve the lifetime of radiation detectors of this kind, the present invention therefore proposes the placing, between the converter and the photosensitive sensor, of a barrier of material impermeable to at least one corrosive chemical species for the sensor and capable of being released during at least one chemical reaction in the converter.
More specifically, the radiation detector according to the invention has a Solid state photosensitive detector comprising a solid-state photosensitive sensor combined with a converter intended to convert a radiation to be detected into a radiation to which the photosensitive sensor is sensitive, the photosensitive sensor comprising one or more photosensitive elements linked to conductors and a passivation layer covering the photosensitive elements and the conductors in order to protect them, characterized in that it comprises, between the passivation layer and the converter, a barrier impermeable to at least one chemical species that is corrosive for the sensor,capable of being released by the converteur during at least one chemical reaction.
The reaction that is likely to occur is a reaction of oxidation and/or a reaction of hydrolysis and/or a reaction of electrolysis.
Preferably, the barrier is chosen to be hydrophobic so as not to accentuate the phenomena of degradation and migration and so not to corrode the sensor if it is deposited directly on it.
The barrier has a refraction index as close as possible to that of the passivation layer.
It is ensured that the barrier closely fits the surface on which it is deposited especially if this surface has raised features.
If the barrier has a sufficiently flat foundation, it may have a chemically inert protection layer on the surface, based for example on fluoride.
The barrier may be made of a resin base such as acrylic resin, polyimide resin, or a resin that is a derivative of benzo-cyclo-butene.
It is also possible to choose a bi-constituent silicone elastomer containing as little solvent as possible after polymerization.
The barrier can also be made out of polyparaxylene or one of its halogen derivatives such as polytetrafluoroparaxylene.
It is also possible to choose a tropicalizing varnish, a sol-gel with at least one mineral constituent such as silica.
The barrier may be made out of a solution based on soluble silicate or at least one bonded polyester membrane.
Diamond carbon deposited in chemical vapor phase also gives good results.
To further increase the protection, it is preferable that the barrier should be formed by a stack of layers.